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Lake Harriet has a nice bandshell with free concerts and movie screenings throughout the summer and fall http://www.minneapolisparks.org/default.asp?PageID=52&prid=1354 Also, the Oriental Garden is nearby, which is amazing, and I don’t like looking at flowers. This is an especially good idea for a date. You can walk/drive to many local restaurants and bars in the uptown area before or after.
Cheap and fun things to do around Minneapolis/St.Paul